Switching voltage regulators are well known. In one type of step down switching regulator, a high side switching transistor, connected to a power supply voltage, is switched between being fully on and fully off at a controlled duty cycle and at a fixed frequency, where the duty cycle is adjusted to maintain the output voltage at a specified regulated voltage. The pulsed current from the switching of the transistor is smoothed by a filter typically consisting of a series inductor and a capacitor connected to ground. A diode or synchronous rectifier is in series with the high side transistor and is typically connected to ground. When the high side transistor is off, the current through the energized inductor is conducted by either the diode or the synchronous rectifier. The output capacitor smoothes the triangular inductor current to supply a regulated DC voltage to the load. The average inductor current equals the current flowing into the load.
Many other types of switching regulators are known, such as a regulator using a fixed on-time of the transistor while varying the interval between the on-times.
At medium and high load currents, a switching regulator is very efficient because the switching transistors have very high conductivity when switched on. For example, to fully turn on a high side PMOS transistor, its gate may be connected to ground to achieve a high gate-source voltage (Vgs), and to fully turn off the transistor, its gate may be applied to the power supply voltage Vin so that Vgs is zero. The gate voltage applied to an NMOS low side switch may also be zero volts or Vin to fully turn off or on the NMOS transistor. Since the transistors have a low resistance when on, there is minimal power dissipation in the transistors.
The gates of MOSFETs have a capacitance. When the regulator is supplying medium and high currents to the load, power wasted by charging and discharging the gate(s) at the switching frequency is a trivial component of the overall power used by the regulator and load.
However, at very light load currents, such as when the load is in a standby mode, the power wasted by charging and discharging the gate(s) at the switching frequency is a significant component of the overall power used by the regulator and load. This problem is exacerbated by the fact that loads are frequently in a low power mode for relatively long periods of time. When the power supply is a battery, it is important to prolong the use time of the battery.
It is known to place the regulator into an intermittent-operation mode at light loads. Such a regulator detects that the load current has gone below a current threshold and shuts down the high side transistor until the output voltage has decayed below an output voltage threshold. During the time when the high side transistor is shut down, the output filter capacitor supplies the current to the load. The length of time that the output voltage decays to the threshold voltage depends on the load current. Once the output voltage has decayed to the threshold voltage, the regulator resumes normal operation (a burst of switching cycles) to raise the output voltage to a certain voltage, typically slightly above the nominal regulated voltage, and the regulator goes into its shut down mode again.
Other forms of low load current modes include a mode where the high side transistor is switched at a variable frequency but at a fixed low duty cycle to keep the output voltage within a range of voltages.
In the known types of intermittent-operation mode techniques, the switching transistor is always controlled to be either fully on or fully off. Any switching of the transistor wastes power by the charging and discharging of its gate.
Bipolar transistors also have a parasitic capacitance, where there is wasted power by the switching on and off of the bipolar transistors by the regulator.
It is desirable to even further reduce the power wasted by a regulator when supplying light load currents.